In order to achieve reliable transmission at radio frequencies from a transmitter to a distant receiver, a power amplifier is needed at the transmitter to increase the power level of a transmitted signal. Generally, a power supply provides supply voltage to the power amplifier. The power amplifier tends to consume a significant amount of battery energy and, as one solution for the excessive power consumption, a switched mode power supply (SMPS) is introduced. By using the SMPS, the battery life can be extended since the power amplifier can be controlled by the SMPS. In particular, a switching arrangement of the SMPS may connect the battery voltage to the power amplifier at the output of the SMPS in a controlled manner. The switching arrangement may switch the supply voltage to the power amplifier between on- and off-states, thus extending the battery life. The time-varying voltage output of the SMPS is applied as a supply voltage to the power amplifier. The SMPS operates on a periodic cycle and consequently the energy transfer is discontinuous. As a result, the output voltage of the SMPS exhibits distortions, such as a ripple. Since the varying output voltage of the SMPS is fed as supply voltage to the power amplifier, the output of the power amplifier, i.e., the transmitted radio frequency signal, may be distorted. Distortion causes, e.g., unwanted modulation of the transmitted signal, deteriorated signal quality and out-of-band spurious tones. In addition, ripple is not the only effect causing variation in the output voltage of the SMPS. For example, the load at the power amplifier (low/high input signal level) is a factor that may cause distortions at the output voltage of the SMPS.
There have been efforts in minimizing the ripple at the output voltage of the power supply by keeping the ripple below a given threshold. Selecting or adjusting the SMPS switching frequency such that the unwanted modulation caused by the ripple does not fall into desired frequency band or filtering out the ripple frequency are techniques that are currently used to combat the voltage variation.
In addition, distortions in the transmitted radio frequency signal may be corrected prior to the power amplifier. However, currently this technique does not take the distortions at the supply voltage, such as ripple, into account. Since these techniques do not completely remove the unwanted modulation of the transmitted signal, novel methods are needed that work against several imperfections of the power supply and, thus, correct the distortions in the transmitted radio frequency signal.